Always add a null function at table index zero

This allows function references (a table index internally) to be nullable

tests/compiler/class-overloading.untouched.wat

@@ -2,6 +2,8 @@
  (type $iv (func (param i32)))
  (type $v (func))
  (global $HEAP_BASE i32 (i32.const 8))
+ (table 1 1 anyfunc)
+ (elem (i32.const 0) $null)
  (memory $0 0)
  (export "memory" (memory $0))
  (export "test" (func $class-overloading/test))
@@ -19,4 +21,6 @@
    (i32.const 0)
   )
  )
+ (func $null (; 3 ;) (type $v)
+ )
 )